Room Temperature Magnetism in Layered Double Hydroxides due to Magnetic Nanoparticles

Some recent reports claiming room temperature spontaneous magnetization in layered double hydroxides (LDHs) have been published; however, the reported materials cause serious concern as to whether this cooperative magnetic behavior comes from extrinsic sources, such as spinel iron oxide nanoparticles. The syntheses of crystalline Fe³⁺-based LDHs with and without impurities have been developed, highlighting the care that must be taken during the synthetic process in order to avoid misidentification of magnetic LDHs

Combination of Magnetic Susceptibility and Electron Paramagnetic Resonance to Monitor the 1D to 2D Solid State Transformation in Flexible Metal–Organic Frameworks of Co(II) and Zn(II) with 1,4-Bis(triazol-1-ylmethyl)benzene

Two families of coordination polymers, {[M­(btix)₂(OH₂)₂]·2NO₃·2H₂O}_<i>n</i> [M = Co (<b>1</b>), Zn (<b>2</b>), Co–Zn (<b>3</b>); btix = 1,4-bis­(triazol-1-ylmethyl)­benzene] and {[M­(btix)₂(NO₃)₂]}_<i>n</i> [M = Co (<b>4</b>), Zn (<b>5</b>), Co–Zn (<b>6</b>)], have been synthesized and characterized. The two conformations of the ligand, <i>syn</i> and <i>anti</i>, lead to one-dimensional (1D) cationic chains or two-dimensional (2D) neutral grids. Extrusion of the water molecules of the 1D compounds results in an irreversible transformation into the 2D compounds, which involves a change in conformation of the btix ligands and a rearrangement in the metal environment with cleavage and reformation of covalent bonds. This structural transformation has been followed by electron paramagnetic resonance (EPR) and magnetic susceptibility measurements to monitor the minor modifications that the metal centers suffer

Combination of Magnetic Susceptibility and Electron Paramagnetic Resonance to Monitor the 1D to 2D Solid State Transformation in Flexible Metal–Organic Frameworks of Co(II) and Zn(II) with 1,4-Bis(triazol-1-ylmethyl)benzene

Two families of coordination polymers, {[M­(btix)₂(OH₂)₂]·2NO₃·2H₂O}_<i>n</i> [M = Co (<b>1</b>), Zn (<b>2</b>), Co–Zn (<b>3</b>); btix = 1,4-bis­(triazol-1-ylmethyl)­benzene] and {[M­(btix)₂(NO₃)₂]}_<i>n</i> [M = Co (<b>4</b>), Zn (<b>5</b>), Co–Zn (<b>6</b>)], have been synthesized and characterized. The two conformations of the ligand, <i>syn</i> and <i>anti</i>, lead to one-dimensional (1D) cationic chains or two-dimensional (2D) neutral grids. Extrusion of the water molecules of the 1D compounds results in an irreversible transformation into the 2D compounds, which involves a change in conformation of the btix ligands and a rearrangement in the metal environment with cleavage and reformation of covalent bonds. This structural transformation has been followed by electron paramagnetic resonance (EPR) and magnetic susceptibility measurements to monitor the minor modifications that the metal centers suffer

Hybrid Materials Based on Magnetic Layered Double Hydroxides: A Molecular Perspective

ConspectusDesign of functional hybrids lies at the very core of synthetic chemistry as it has enabled the development of an unlimited number of solids displaying unprecedented or even improved properties built upon the association at the molecular level of quite disparate components by chemical design. Multifunctional hybrids are a particularly appealing case among hybrid organic/inorganic materials. Here, chemical knowledge is used to deploy molecular components bearing different functionalities within a single solid so that these properties can coexist or event interact leading to unprecedented phenomena. From a molecular perspective, this can be done either by controlled assembly of organic/inorganic molecular tectons into an extended architecture of hybrid nature or by intercalation of organic moieties within the empty channels or interlamellar space offered by inorganic solids with three-dimensional (MOFs, zeolites, and mesoporous hosts) or layered structures (phosphates, silicates, metal dichalcogenides, or anionic clays).This Account specifically illustrates the use of layered double hydroxides (LDHs) in the preparation of magnetic hybrids, in line with the development of soft inorganic chemistry processes (also called “Chimie Douce”), which has significantly contributed to boost the preparation hybrid materials based on solid-state hosts and subsequent development of applications. Several features sustain the importance of LDHs in this context. Their magnetism can be manipulated at a molecular level by adequate choice of constituting metals and interlayer separation for tuning the nature and extent of magnetic interactions across and between planes. They display unparalleled versatility in accommodating a broad range of anionic species in their interlamellar space that encompasses not only simple anions but chemical systems of increasing dimensionality and functionalities. Their swelling characteristics allow for their exfoliation in organic solvents with high dielectric strength, to produce two-dimensional nanosheets with atomic thickness that can be used as macromolecular building blocks in the assembly of nanocomposites.We describe how these advantageous properties turn LDHs into excellent vehicles for the preparation of multifunctional materials with increasing levels of complexity. For clarity, the reader will first find a succinct description of the most relevant aspects controlling the magnetism of LDHs followed by their use in the preparation of magnetic hybrids from a molecular perspective. This includes the intercalation anionic species of increasing nuclearity like paramagnetic mononuclear complexes, stimulus-responsive molecular guests, one- and two-dimensional coordination polymers, or even preassembled 2D networks. This approach allows us to evolve from “dual-function” materials with coexistence, for example, of magnetism and superconductivity, to smart materials in which the magnetic or structural properties of the LDH layers can be tuned by applying an external stimulus like light or temperature. We will conclude with a brief look into the promising features offered by magnetic nanocomposites based on LDHs and our views on the most promising directions to be pursued in this context

Iron(II) complex of 2-(1H-pyrazol-1-yl)pyridine-4-carboxylic acid (ppCOOH) suitable for surface deposition

<p>The synthesis, structural and magnetic characterization of the tris iron(II) complex of 2-(1H-pyrazol-1-yl)pyridine-4-carboxylic acid (ppCOOH) ligand are reported in [Fe(ppCOOH)₃](ClO₄)₂·0.5H₂O·2EtOH. Single crystal structure and magnetic characterization of the bulk compound show that the low-spin state is dominant from 2 to 400 K. ESI-MS and UV–Vis spectroscopy experiments indicate that acetonitrile solutions of this complex are stable with time. ESI-MS confirms the presence of the tris complex in solution. This complex can be deposited onto SiO₂ surfaces due to the presence of carboxylic acid groups by immersing the substrates into acetonitrile solutions of the complex. XPS spectra of the deposited complex are similar to those of the bulk sample. AFM images show a slight increase in roughness with respect to the naked substrate and the absence of aggregates. These results are consistent with the formation of a monolayer of the complex on the surface.</p

A Mixed-Ligand Approach for Spin-Crossover Modulation in a Linear Fe^II Coordination Polymer

In this work, we present a family of Fe^II coordination polymers of general formula [Fe­(btzx)_{3–3<i>x</i>}(btix)_3<i>x</i>]­(ClO₄)₂ with interesting spin-crossover properties. These coordination polymers have been synthesized using chemical mixtures of two different but closely related ligands, 1,4-bis­(tetrazol-1-ylmethyl)­benzene (btzx) and 1,4-bis­(triazol-1-ylmethyl)­benzene (btix), and the effect of a gradual substitution of the ligand in the spin transition temperature has been investigated. Several chemical mixtures have been structurally characterized by X-ray powder diffraction indicating a clear critical amount in the composition of the mixture after which mixed phases rather than a single phase comprising mixed components are observed. Importantly, this approach causes the appearance of a new transition at lower temperatures that is not present in the pure [Fe­(L)₃]­(ClO₄)₂ systems

The Series of Molecular Conductors and Superconductors ET₄[AFe(C₂O₄)₃]·PhX (ET = bis(ethylenedithio)tetrathiafulvalene; (C₂O₄)^2– = oxalate; A⁺ = H₃O⁺, K⁺; X = F, Cl, Br, and I): Influence of the Halobenzene Guest Molecules on the Crystal Structure and Superconducting Properties

An extensive series of radical salts formed by the organic donor bis­(ethylenedithio)­tetrathiafulvalene (ET), the paramagnetic tris­(oxalato)­ferrate­(III) anion [Fe­(C₂O₄)₃]^3–, and halobenzene guest molecules has been synthesized and characterized. The change of the halogen atom in this series has allowed the study of the effect of the size and charge polarization on the crystal structures and physical properties while keeping the geometry of the guest molecule. The general formula of the salts is ET₄[A^IFe­(C₂O₄)₃]·G with A/G = H₃O⁺/PhF (<b>1</b>); H₃O⁺/PhCl (<b>2</b>); H₃O⁺/PhBr (<b>3</b>), and K⁺/PhI (<b>4</b>), (crystal data at room temperature: (<b>1</b>) monoclinic, space group <i>C</i>2/<i>c</i> with <i>a</i> = 10.3123(2) Å, <i>b</i> = 20.0205(3) Å, <i>c</i> = 35.2732(4) Å, β = 92.511(2)°, <i>V</i> = 7275.4(2) ų, <i>Z</i> = 4; (<b>2</b>) monoclinic, space group <i>C</i>2/<i>c</i> with <i>a</i> = 10.2899(4) Å, <i>b</i> = 20.026(10) Å, <i>c</i> = 35.411(10) Å, β = 92.974°, <i>V</i> = 7287(4) ų, <i>Z</i> = 4; (<b>3</b>) monoclinic, space group <i>C</i>2/<i>c</i> with <i>a</i> = 10.2875(3) Å, <i>b</i> = 20.0546(15) Å, <i>c</i> = 35.513(2) Å, β = 93.238(5)°, <i>V</i> = 7315.0(7) ų, <i>Z</i> = 4; (<b>4</b>) monoclinic, space group <i>C</i>2/<i>c</i> with <i>a</i> = 10.2260(2) Å, <i>b</i> = 19.9234(2) Å, <i>c</i> = 35.9064(6) Å, β = 93.3664(6)°, <i>V</i> = 7302.83(18) ų, <i>Z</i> = 4). The crystal structures at 120 K evidence that compounds <b>1</b>–<b>3</b> undergo a structural transition to a lower symmetry phase when the temperature is lowered (crystal data at 120 K: (<b>1</b>) triclinic, space group <i>P</i>1̅ with <i>a</i> = 10.2595(3) Å, <i>b</i> = 11.1403(3) Å, <i>c</i> = 34.9516(9) Å, α = 89.149(2)°, β = 86.762(2)°, γ = 62.578(3)°, <i>V</i> = 3539.96(19) ų, <i>Z</i> = 2; (<b>2</b>) triclinic, space group <i>P</i>1̅ with <i>a</i> = 10.25276(14) Å, <i>b</i> = 11.15081(13) Å, <i>c</i> = 35.1363(5) Å, α = 89.0829(10)°, β = 86.5203(11)°, γ = 62.6678(13)°, <i>V</i> = 3561.65(8) ų, <i>Z</i> = 2; (<b>3</b>) triclinic, space group <i>P</i>1̅ with <i>a</i> = 10.25554(17) Å, <i>b</i> = 11.16966(18) Å, <i>c</i> = 35.1997(5) Å, α = 62.7251(16)°, β = 86.3083(12)°, γ = 62.7251(16)°, <i>V</i> = 3575.99(10) ų, <i>Z</i> = 2; (<b>4</b>) monoclinic, space group <i>C</i>2/<i>c</i> with <i>a</i> = 10.1637(3) Å, <i>b</i> = 19.7251(6) Å, <i>c</i> = 35.6405(11) Å, β = 93.895(3)°, <i>V</i> = 7128.7(4) ų, <i>Z</i> = 4). A detailed crystallographic study shows a change in the symmetry of the crystal for compound <b>3</b> at about 200 K. This structural transition arises from the partial ordering of some ethylene groups in the ET molecules and involves a slight movement of the halobenzene guest molecules (which occupy hexagonal cavities in the anionic layers) toward one of the adjacent organic layers, giving rise to two nonequivalent organic layers at 120 K (compared to only one at room temperature). The structural transition at about 200 K is also observed in the electrical properties of <b>1</b>–<b>3</b> and in the magnetic properties of <b>1</b>. The direct current (dc) conductivity shows metallic behavior in salts <b>1</b>–<b>3</b> with superconducting transitions at about 4.0 and 1.0 K in salts <b>3</b> and <b>1</b>, respectively. Salt <b>4</b> shows a semiconductor behavior in the temperature range 300–50 K with an activation energy of 64 meV. The magnetic measurements confirm the presence of high spin <i>S</i> = 5/2 [Fe­(C₂O₄)₃]^3– isolated monomers together with a Pauli paramagnetism, typical of metals, in compounds <b>1</b>–<b>3</b>. The magnetic properties can be very well reproduced in the whole temperature range with a simple model of isolated <i>S</i> = 5/2 ions with a zero field splitting plus a temperature independent paramagnetism (Nα) with the following parameters: <i>g</i> = 1.965, |<i>D</i>| = 0.31 cm^–1, and Nα = 1.5 × 10^–3 emu mol^–1 for <b>1</b>, <i>g</i> = 2.024, |<i>D</i>| = 0.65 cm^–1, and Nα = 1.4 × 10^–3 emu mol^–1 for <b>2</b>, and <i>g</i> = 2.001, |<i>D</i>| = 0.52 cm^–1, and Nα = 1.5 × 10^–3 emu mol^–1 for <b>3</b>

The Series of Molecular Conductors and Superconductors ET₄[AFe(C₂O₄)₃]·PhX (ET = bis(ethylenedithio)tetrathiafulvalene; (C₂O₄)^2– = oxalate; A⁺ = H₃O⁺, K⁺; X = F, Cl, Br, and I): Influence of the Halobenzene Guest Molecules on the Crystal Structure and Superconducting Properties

An extensive series of radical salts formed by the organic donor bis­(ethylenedithio)­tetrathiafulvalene (ET), the paramagnetic tris­(oxalato)­ferrate­(III) anion [Fe­(C₂O₄)₃]^3–, and halobenzene guest molecules has been synthesized and characterized. The change of the halogen atom in this series has allowed the study of the effect of the size and charge polarization on the crystal structures and physical properties while keeping the geometry of the guest molecule. The general formula of the salts is ET₄[A^IFe­(C₂O₄)₃]·G with A/G = H₃O⁺/PhF (<b>1</b>); H₃O⁺/PhCl (<b>2</b>); H₃O⁺/PhBr (<b>3</b>), and K⁺/PhI (<b>4</b>), (crystal data at room temperature: (<b>1</b>) monoclinic, space group <i>C</i>2/<i>c</i> with <i>a</i> = 10.3123(2) Å, <i>b</i> = 20.0205(3) Å, <i>c</i> = 35.2732(4) Å, β = 92.511(2)°, <i>V</i> = 7275.4(2) ų, <i>Z</i> = 4; (<b>2</b>) monoclinic, space group <i>C</i>2/<i>c</i> with <i>a</i> = 10.2899(4) Å, <i>b</i> = 20.026(10) Å, <i>c</i> = 35.411(10) Å, β = 92.974°, <i>V</i> = 7287(4) ų, <i>Z</i> = 4; (<b>3</b>) monoclinic, space group <i>C</i>2/<i>c</i> with <i>a</i> = 10.2875(3) Å, <i>b</i> = 20.0546(15) Å, <i>c</i> = 35.513(2) Å, β = 93.238(5)°, <i>V</i> = 7315.0(7) ų, <i>Z</i> = 4; (<b>4</b>) monoclinic, space group <i>C</i>2/<i>c</i> with <i>a</i> = 10.2260(2) Å, <i>b</i> = 19.9234(2) Å, <i>c</i> = 35.9064(6) Å, β = 93.3664(6)°, <i>V</i> = 7302.83(18) ų, <i>Z</i> = 4). The crystal structures at 120 K evidence that compounds <b>1</b>–<b>3</b> undergo a structural transition to a lower symmetry phase when the temperature is lowered (crystal data at 120 K: (<b>1</b>) triclinic, space group <i>P</i>1̅ with <i>a</i> = 10.2595(3) Å, <i>b</i> = 11.1403(3) Å, <i>c</i> = 34.9516(9) Å, α = 89.149(2)°, β = 86.762(2)°, γ = 62.578(3)°, <i>V</i> = 3539.96(19) ų, <i>Z</i> = 2; (<b>2</b>) triclinic, space group <i>P</i>1̅ with <i>a</i> = 10.25276(14) Å, <i>b</i> = 11.15081(13) Å, <i>c</i> = 35.1363(5) Å, α = 89.0829(10)°, β = 86.5203(11)°, γ = 62.6678(13)°, <i>V</i> = 3561.65(8) ų, <i>Z</i> = 2; (<b>3</b>) triclinic, space group <i>P</i>1̅ with <i>a</i> = 10.25554(17) Å, <i>b</i> = 11.16966(18) Å, <i>c</i> = 35.1997(5) Å, α = 62.7251(16)°, β = 86.3083(12)°, γ = 62.7251(16)°, <i>V</i> = 3575.99(10) ų, <i>Z</i> = 2; (<b>4</b>) monoclinic, space group <i>C</i>2/<i>c</i> with <i>a</i> = 10.1637(3) Å, <i>b</i> = 19.7251(6) Å, <i>c</i> = 35.6405(11) Å, β = 93.895(3)°, <i>V</i> = 7128.7(4) ų, <i>Z</i> = 4). A detailed crystallographic study shows a change in the symmetry of the crystal for compound <b>3</b> at about 200 K. This structural transition arises from the partial ordering of some ethylene groups in the ET molecules and involves a slight movement of the halobenzene guest molecules (which occupy hexagonal cavities in the anionic layers) toward one of the adjacent organic layers, giving rise to two nonequivalent organic layers at 120 K (compared to only one at room temperature). The structural transition at about 200 K is also observed in the electrical properties of <b>1</b>–<b>3</b> and in the magnetic properties of <b>1</b>. The direct current (dc) conductivity shows metallic behavior in salts <b>1</b>–<b>3</b> with superconducting transitions at about 4.0 and 1.0 K in salts <b>3</b> and <b>1</b>, respectively. Salt <b>4</b> shows a semiconductor behavior in the temperature range 300–50 K with an activation energy of 64 meV. The magnetic measurements confirm the presence of high spin <i>S</i> = 5/2 [Fe­(C₂O₄)₃]^3– isolated monomers together with a Pauli paramagnetism, typical of metals, in compounds <b>1</b>–<b>3</b>. The magnetic properties can be very well reproduced in the whole temperature range with a simple model of isolated <i>S</i> = 5/2 ions with a zero field splitting plus a temperature independent paramagnetism (Nα) with the following parameters: <i>g</i> = 1.965, |<i>D</i>| = 0.31 cm^–1, and Nα = 1.5 × 10^–3 emu mol^–1 for <b>1</b>, <i>g</i> = 2.024, |<i>D</i>| = 0.65 cm^–1, and Nα = 1.4 × 10^–3 emu mol^–1 for <b>2</b>, and <i>g</i> = 2.001, |<i>D</i>| = 0.52 cm^–1, and Nα = 1.5 × 10^–3 emu mol^–1 for <b>3</b>

Solvent-Free Synthesis of a Pillared Three-Dimensional Coordination Polymer with Magnetic Ordering

A new magnetic coordination polymer, [Fe­(bipy)­(im)₂] (bipy = 4,4-bipyridine and im = imidazole), has been synthesized in a solvent-free reaction. Structural analysis reveals a pillared 3D coordination polymer composed by neutral layers, formed by iron­(II) and imidazolate linkers, interconnected by bipy ligands which serve as pillars. Magnetic measurements show that the material magnetically orders at low temperatures (<i>T</i>_c = 14.5 K) as a weak ferromagnet, likely due to a spin canting

Cobalt Clusters with Cubane-Type Topologies Based on Trivacant Polyoxometalate Ligands

Four novel cobalt-substituted polyoxometalates having cobalt cores exhibiting cubane or dicubane topologies have been synthesized and characterized by IR, elemental analysis, electrochemistry, UV–vis spectroscopy, X-ray single-crystal analysis, and magnetic studies. The tetracobalt­(II)-substituted polyoxometalate [Co₄(OH)₃­(H₂O)₆­(PW₉O₃₄)]^4– (<b>1</b>) consists of a trilacunary [B-α-PW₉O₃₄]^9– unit which accommodates a cubane-like {Co^II₄O₄} core. In the heptacobalt­(II,III)-containing polyoxometalates [Co₇(OH)₆­(H₂O)₆­(PW₉O₃₄)₂]^9– (<b>2</b>), [Co₇(OH)₆­(H₂O)₄­(PW₉O₃₄)₂]_<i>n</i>^9<i>n</i>– (<b>3</b>), and [Co₇(OH)₆­(H₂O)₆­(P₂W₁₅O₅₆)₂]^15– (<b>4</b>), dicubane-like {Co^II₆Co^IIIO₈} cores are encapsulated between two heptadentate [B-α-PW₉O₃₄]^9– (in <b>2</b> and <b>3</b>) or [α-P₂W₁₅O₅₆]^15– (in <b>4</b>) ligands. While <b>1</b>, <b>2</b>, and <b>4</b> are discrete polyoxometalates, <b>3</b> exhibits a polymeric, chain-like structure that results from the condensation of polyoxoanions of type <b>2</b>. The magnetic properties of these complexes have been fitted according to an anisotropic exchange model in the low-temperature regime and discussed on the basis of ferromagnetic interactions between Co²⁺ ions with angles Co–L–Co (L = O, OH) close to orthogonality and weakly antiferromagnetic interactions between Co²⁺ ions connected through central diamagnetic Co³⁺ ion. Moreover, we will show the interest of the unique spin structures provided by these cubane and dicubane cobalt topologies in molecular spintronics (molecular spins addressed though an electric field) and quantum computing (spin qu-gates)